The alert covering the city and several suburbs was lifted at 11 a.m. PDT.

"Follow-up testing of drinking water has shown the absence of bacterial contamination," said Tim Hall, spokesman for the Portland Water Bureau.

Hall recommended flushing all taps for two minutes, or until the water runs cold, before drinking the water.

"This will flush any potentially contaminated water from the plumbing," he said.

The city issued the boil-water alert Friday morning after repeat water samples in four locations showed the presence of E. coli, a type of bacteria found in feces, and total coliform, bacteria commonly found in the environment but problematic in drinking water, officials said.

Bureau Director David Shaff said it was the largest boil water alert in the city's history, impacting some 670,000 customers. The cities of Tigard, King City and parts of Gresham were also affected.

Tri-County Health Officer Paul Lewis said the E. coli came from animal fecal matter. The most likely symptom from people drinking contaminated water would be diarrhea.

Medical experts say that E. coli can take up to two days to incubate in someone, and symptoms may not show up immediately.

Dr. John Heiser, an emergency room physician at Providence St. Vincent's Medical Center, said the elderly, young children and people with chronic illnesses were most at risk.